The present invention relates to a graded index plastic optical fiber having a small bending loss and being excellent in heat resistance and heat and humidity resistance.
Known as a graded index plastic optical fiber is a plastic optical fiber having a graded index distribution structure comprising, as a matrix, a non-crystalline fluoropolymer having substantially no Cxe2x80x94H bond and a substance having a different refractive index from the matrix, distributed with a concentration gradient in a radial direction (see JP-A-8-5848). Further, JP-A-8-304636 discloses an optical fiber having a polymer with a refractive index lower than matrix, provided on the outer circumference of the matrix, in order to avoid an increase of the attenuation loss by bending in such a graded index optical fiber.
The conventional graded index optical fiber having such a low refractive index polymer provided on the outer circumference, has had a problem that the attenuation loss increases when it is subjected to a heat resistance/heat and humidity resistance tests such as long term heat resistance test (70xc2x0 C. for 1,000 hours), a temperature cycle test (70xc2x0 C./xe2x88x9220xc2x0 C.xc3x9710 times) or a heat and humidity cycle test (65xc2x0 C., humidity of 95%/xe2x88x9210xc2x0 C.xc3x9710 times).
The present inventors have analyzed the fibers after the heat cycle test and the heat and humidity cycle test, and as a result, have found that peeling which takes place between the outer layer made of the low refractive index polymer and the inner layer having a graded index distribution formed, is the cause for an increase of the attenuation loss.
On the basis of the recognition of such problems, the present inventors have conducted an extensive study and as a result have found that in order to improve the adhesion between the inner layer and the outer layer, it is effective to employ a polymer having a high affinity to the polymer constituting the matrix of the inner layer, as the low refractive index material of the outer layer. Namely, the present invention is to provide a graded index optical fiber anew wherein an outer layer is formed outside the inner layer having a graded index formed, by means of a polymer having a refractive index lower than the refractive index of the outermost portion of the inner layer and a good adhesive property, to provide an optical fiber having an increase of the attenuation loss by bending reduced while maintaining the heat resistance and the heat and humidity resistance. The present invention is the following invention based on such a discovery.
A plastic optical fiber which is a graded index optical fiber having a concentric inner/outer at least two layer structure, wherein the inner layer has a graded index structure made of a non-crystalline fluoropolymer (a) having substantially no Cxe2x80x94H bond, and the outer layer has a refractive index lower than the refractive index of the outermost portion of the inner layer and is made of a fluoropolymer material (c) selected from the following 1) and 2):
1) a fluoropolymer (d) containing the same polymerized units as the polymerized units in the fluoropolymer (a), and
2) a mixture (f) of a fluoropolymer (a) with another fluoropolymer (e).
In the optical fiber of the present invention, in order for the fluoropolymer material (c) to have a high adhesive property with the fluoropolymer (a) and in order not to let the heat resistance and the heat and humidity resistance of the optical fiber deteriorate, the glass transition temperature Tgc of the fluoropolymer material (c) is preferably 70xc2x0 C. less than Tgc less than Tga+30xc2x0 C., where Tga is the glass transition temperature of the fluoropolymer (a). Further, the fluoropolymer (d) preferably contains at least 30 mol % of the same polymerized units as the polymerized units in the fluoropolymer (a). Here, the polymerized units in the present invention are meant for repeating units in a polymer formed by a polymerization reaction of a monomer.
Further, in order not to let the attenuation loss of the optical fiber increase, the refractive index of the fluoropolymer material (c) is preferably lower by at least 0.03 than the refractive index of the outermost portion of the inner layer. Here, the refractive index in the present invention is a refractive index against sodium D line spectrum.
In the optical fiber of the present invention, its inner layer is preferably an inner layer which contains the fluoropolymer (a) as a matrix, and a substance (b) having a different refractive index is distributed in the matrix to form the graded index structure. As the fluoropolymer (a), a fluoropolymer having a ring structure in its main chain as disclosed in the above-mentioned prior art is preferred. Likewise, the fluoropolymer (d) and the fluoropolymer (e) are preferably fluoropolymers having ring structures in their main chains. Further, the optical fiber of the present invention preferably has a protective coating layer made of a synthetic resin provided outside the outer layer. As such a synthetic resin, a thermoplastic resin made of a polymer other than the fluoropolymer (a), the fluoropolymer (d) and the fluoropolymer (e), which has heretofore been used or proposed to be used as a protective coating layer for an optical fiber, is preferred.